Michael Rawlins: Statistics can help, but doctors must also use their judgement

Nowhere is the longstanding argument over the choice between inductive and deductive approaches more hotly argued than in the nature of the evidence that should support the use of therapeutic interventions.

The dispute about the evidential basis of modern medicine has become particularly apparent with the emergence of "hierarchies" of evidence. Such hierarchies place randomised controlled trials (RCTs) at their summit with observational studies nestling in the foothills. They are used to provide some intimation of the "strength" of the underlying evidence, and to then "grade" therapeutic recommendations.

But the notion that evidence can be reliably placed in hierarchies is illusory: judgements are an essential part of the decision-making process. As Bradford Hill, the architect of the RCT, stated so cogently: "Any belief that the controlled trial is the only way would mean not that the pendulum had swung too far but that it had come right off the hook."

At root, the problem with hierarchies is that they attempt to replace judgement with an oversimplistic assessment of the quality of the available evidence. But as William Blake observed: "God forbid that truth should be confined to mathematical demonstration."

Decision-makers have to incorporate judgements in reaching their conclusions. Experiment, observation and mathematics have a crucial role to play in providing the evidential basis for modern therapeutics. Arguments about the relative importance of each are an unnecessary distraction. Hierarchies of evidence should be replaced by embracing a diversity of approaches.

This is not a plea to abandon RCTs and replace them with observational studies. Rather, it is a plea to investigators to continue to develop and improve their methodologies; to decision-makers to avoid adopting entrenched positions about the nature of evidence; and for both to accept that the interpretation of evidence requires judgement. It is scientific judgement – conditioned by the totality of the evidence – that lies at the heart of making decisions about the benefits and harms of therapeutic interventions.

Professor Sir Michael Rawlins chairs the National Institute for Health and Clinical Excellence. He will deliver the Harveian Oration at the Royal College of Physicians in London tonight